The present invention relates to a piece-threading device to be used on a numerically controlled lathe.
As is known, in the medical field are conventionally used screws, stems or other mechanical pieces to be screwed into the bone structure of a patient.
At present, the best method used in the prior art for making threaded elements to be used in the medical or dental field, is a specifically designed cutting process, the so-called “swivel” threading process or, in a word-wide manner, the “Thread Whirling Process”.
Prior methods allow to make threaded pieces with a variable pitch thread, a very long pitch thread, highly buttressed threads, bevel threads or variably buttressed threads.
It should be apparent that the selection of the thread will depend on the specific bone structure, the screw must be screwed into.
The prior machining method allowing to make different types of threads on a lathe, also allows to machine, without any technical problems, very hard metals, such as stainless steel, titanium or the like.
From the prior art it is also known that the surface quality of a thread depends on clearances which are present in the kinematic chain driving the tools used for performing the above mentioned thread whirling process.
The prior method for making the above mentioned threads, however, is not a continuous cutting method, but an interrupted cutting operation, like a milling machining operation, and this type of machining greatly stresses the driving means; moreover, because of unavoidable clearances, the surfaces of the resulting threads are inevitably affected by machining defects and rags.
Usually, in prior devices, the threading tools are driven by a driving motor through a series of gears which, because of their nature, necessarily require a minimum clearance, to provide a proper mutual meshing, and this clearance, even if it has a minimum value, negatively affects the precision and quality of the resulting thread surface.
To further limit the above undesired clearance, prior deriving gears have been recently replaced by a belt transmission, designed to allow the motor to rotatively drive the machining tool.
This approach, however, has the drawback that a resilient element in the form of a belt must be used in order to transmit the rotary motion from the driving motor to the machining tool. The use of a belt generates inevitable undesired oscillations in the rotary movement of the tool that must be subjected to continuous control. Moreover, the driving belt must be subjected to periodic controls for its inevitable wear and to repeatedly accomplish the tensioning thereof.
Thus, both the above mentioned gear assembly and driving belt, as conventionally used for rotatively driving prior threading tools, cause an undesired increase of the overall size of the device.
Moreover, it has been also found that in prior threading device, also called “turbothreaders” designed for making a swivel thread, it would be absolutely necessary to rotatively drive the head/spindle assembly for performing the thread helix; moreover, for making some types of threads, it would be indispensable to tilt the threading head with respect to the horizontal plane and, moreover, to provide a tool holder spindle having a conical inner configuration, in order not to impact against the workpiece during the threading operations.